This invention relates to direction finding and more particularly to a system for calibrating an array of direction finding antennas on an aircraft.
Typically and for many years, surveillance aircraft have been provided with an array of for instance sixteen to thirty-two loop and or monopole-type antennas dispersed about the surface of the aircraft to be able to get the bearing line from this aircraft to a source of electromagnetic radiation. This source can be from for instance transmitters used by enemy troops, transmission sources associated with weapons and ordinance, or can be radiation from any type of communications device.
In the past, surveillance aircraft with such an array of direction finding antennas have been calibrated by establishing a calibration antenna on the ground and flying at some distance from this antenna so that the depression angle between the aircraft and the antenna is close to 0xc2x0. By depression angle is meant the angle down from the horizontal of a bearing line between the plane and a radiation source on the ground. The calibration of the antenna array involved the flying of an aircraft in a horizontal circular or banana pattern such that the aircraft was in essence turned 360xc2x0 in azimuth, with measurements made of the response of the antennas at 1xc2x0 or 20xc2x0 azimuth increments and for all of the frequencies of interest. This provided a data set so that actual measurements from the aircraft could be correlated with the calibrated data set in order to arrive at a precise bearing line from the source of the electromagnetic radiation to the aircraft. In one example, the desired accuracy was 5xc2x0.
As was usually the case, these surveillance aircraft operated a fairly large distance away from enemy territory for safety reasons. Thus, the signals coming from enemy radios or transmitters would come in at a relatively shallow depression angle.
However, with the use of unmanned aerial vehicles, or UAV""S, due to the fact that they are unmanned, they can be flown directly over enemy territory unlike the manned surveillance aircraft used previously. The reason for using unmanned aircraft is to limit the exposure of airmen to hostile fire. However, the use of such UAV""s requires that the antenna arrays on the UAV""s be calibrated for all depression angles including the relatively deep depression 80xc2x0-90xc2x0 angles that exist as the UAV flies directly over a surveilled area.
The problem of utilizing a full-scale airplane and flying it over a calibrating antenna is that it is very difficult for a plane to maintain a constant depression angle relative to the calibrating antenna when flying the aircraft in a circle. The reason is that it is not possible to spin the aircraft 360xc2x0 on its own axes above the ground in order to get calibration data for all azimuths. Rather the plane can only execute a relatively large circle or oval. If the plane is close to the calibration antenna, the depression angle at the nearest point on the circle varies greatly from the depression angle at the far point of the circle. Thus, it is exceeding difficult to maintain a constant depression angle for a 360xc2x0 azimuth sweep when flying a full-scale aircraft. This is due to the dynamics of flight which prohibit tight turns.
In short, when trying to calibrate a DF antenna array at a constant deep depression angle, one cannot do it by flying a plane.
Noting that there is a difficulty of rotating an aircraft 360xc2x0 while maintaining a predetermined depression angle for calibration purposes, in the subject invention, an electrically similar scale model of the aircraft is provided with antennas at the same positions as they are on the full-scale aircraft. An optimization technique adjusts the response of the antennas on the model to the expected outputs of the antennas on the full-scale platform. This scale model is located on, the ground at a calibration range and is supported by a gantry which rotates the model over a number of depression angles and also swings the model over the full 360xc2x0 azimuth range that is required. Measurements are then taken from the model at a wide variety of depression angles, one of which is identical to the shallow depression angle of the full-scale aircraft executing maneuvers at a distance from the calibration antenna. The depression angle measurements from the full-scale aircraft are made at quite some distance from the calibration antenna so that, for instance, a nearly constant depression angle in the range of xe2x88x922xc2x0 to xe2x88x925xc2x0 can be obtained. The plane is flown in a pattern that will establish the response of the antennas in a 360xc2x0 azmuth sweep for 1xc2x0 increments and for all of the frequencies of interest. This provides a data set for the full-scale platform and the particular antenna array, which is then used as a base line to be able to correlate the results of the model with the full-scale aircraft.
Data collected from the model at this shallow depression angle for the indicated frequencies, and at 1xc2x0 azimuth increments when processed with live data from the aircraft at this shallow depression angle results in a set of complex weights which are used to account for differences between the full-scale and model antenna responses.
Once having model data for this shallow depression angle, data is then taken from them model at the other desired depression angles. This data is corrected by the weights derived from the model and the full-scale aircraft at the above-mentioned shallow depression angle. It is thus a finding of the subject invention that weights generated for the single shallow depression angle done in this fashion can be used to adjust and correct the model airborne array data recorded for all depression angles.
The model therefore provides virtually all of the data that is to be used in the full-scale aircraft. The result is that the full-scale aircraft will be provided with a data set or array manifold that permits accurate direction finding when the aircraft is flying at stand off or stand-in ranges from electromagnetic sources.
Thus, in the present invention, live data need only be taken at one depression angle, which data is then compared with data at a number of different depression angles taken from the model. With the advent of airborne vehicles that fly directly over hostile territory for detecting the direction of RF sources, a method for calibrating the antennas on the vehicles is provided so as to correctly determine the direction of the source of electromagnetic radiation, especially at deep depression angles associated with such flights. In order to accomplish this, all that is required is to obtain a set of data from a given relatively shallow depression angle in a flight test and then provide a model of the aircraft with antennas appropriately located. A weighting system is then devised to be able to weight the outputs of the various antennas on the model such that a data set or array manifold is available at the aircraft to correct the output of the airborne antenna array. When a direction finding algorithm is applied, the accuracy of the direction finding result will be within specified accuracy requirements.
Note that a complex optimization technique is used to generate complex weights that are then used to adjust the data collected from the model to account for the differences between the full-scale and the model antennas arrays. The result is an easily obtained deep depression calibration database.
In summary, a system for calibrating airborne direction finding antenna arrays eliminates the problem of trying to maintain a constant depression angle when flying an airplane directly over a calibration source antenna to collect deep depression angle data. The deep depression angle data necessary for calibration is provided by data from a scale model of the aircraft having a direction-finding array which simulates the actual direction-finding array on the aircraft. In order to collect deep depression angle data, the model is pivoted through 360xc2x0 while maintaining a controlled depression angle. Thus, it is unnecessary for calibration to actually fly a plane to attempt to obtain deep depression angle measurements. In the subject system, only a very small set of data is required from the aircraft. Thus, with the exception of some baseline shallow elevation angle data from this plane, the calibration data comes strictly from the scale model which is much more easily obtained. Optimization techniques are used in which a set of data is collected from the airplane at one shallow depression angle which is used with the data collected from the scale model at this shallow depression angle to derive a complex set of optimized weights that are then applied to the data collected from the model at the remainder of the depression angles to obtain the appropriate database for use on this aircraft for direction finding. In so doing, the aircraft need only be flown to establish data at a relatively shallow depression angle which can be easily collected by an aircraft flying in circles at some distance from the calibration source.